1. Field of the invention PA1 2. Description of Related Art
The present invention relates to an image forming apparatus being used for a copier, a printer and or the like, and more particularly, to an image forming apparatus capable of generating an electric field in order to direct charged toner particles to form an image on a recording medium.
Conventionally, there is an image forming apparatus disclosed in the U.S. Pat. No. 3,689,935. In the image forming apparatus, an aperture electrode having a plurality of small holes or apertures, is provided. The aperture electrode controls the passage of toner particles through each of the apertures upon being applied with a driving signal based on an image signal. Then, those toner particles which pass through each of the apertures are held on a supporting member, so that an image can be recorded on the supporting member, such as a recording paper. In the image forming apparatus of the type described, the aperture electrode comprises an insulative layer, a reference electrode and a plurality of control electrodes. The reference electrode is made of one sheet forming an electric conductor layer on one side of the insulative layer. The control electrodes are formed independently of each other on the other side of the insulative layer, in the longitudinal direction of the insulative layer. Moreover, each of the control electrodes has at least one circular aperture. In other words, the aperture penetrates the above-mentioned three layers (the reference electrode, the insulative layer and the control electrode).
The image forming apparatus further comprises a device for supplying charged toner particles, a control electrode driving circuit for applying electric potential between the control electrode and the reference electrode, a device for moving the supporting member where an image is to be formed.
In the above-mentioned image forming apparatus, the charged toner particles are supplied to the aperture of the control electrodes. Selected ones of the control electrodes receive an electric potential supplied by the control electrode driving circuit, so that a flow of those toner particles increases speed on passing through each of the apertures. Then, those toner particles can pass through the aperture. The toner particles pass through the aperture in a predetermined short time and adhere to the supporting member in the form of a dot. Thus, a plurality of dots are formed on the supporting member based on image data, so that a desired image is formed.
However, each of the apertures provided in the aperture electrode of the above-described image forming apparatus has a circular shape. Therefore, when a plurality of dots composed of toner particles are formed on the supporting member in order to form the image, an interval between dots adjacent to each other in the direction of movement of the supporting member gets narrow, so that there is a problem in that the resolution of the image forming apparatus decreases.
Parts of a dot corresponding to rise and fall of an image signal, that is, both ends of the dot in the direction of movement of the supporting member have semicircular shapes corresponding to the circular aperture. Since each of the diameter portions of those semicircles corresponds to the rise and the fall of the image signal, those semicircles in both ends of the dot in the direction of movement of the supporting member are surplus area. Therefore, each of the dots does not correspond to the image signal accurately. Therefore, when a space where a dot is not formed is small, that is, a time between a fall of the image signal and a rise of the next image signal is short, the interval between dots adjacent to each other gets narrow. So that, the resolution of the image forming apparatus decreases.